Slow Shutter App + Waterfalls
This cool app is available through iTunes. With The Slow Shutter App, you can adjust the following via the "gear" icon located at the bottom left of the screen (see image on right):
Setting this to "MAXIMUM" will give you the most "flow" effect on running water.
Adjust this from MINIMUM to BULB (leaving the "shutter" open until you click twice). The longer the time (15s, 30s), the more of an effect you receive, but you'll need a good tripod. If you don't have one handy -- use a rock and your headphones to take the photo.
Leave on AUTO, or adjust to the level of brightness (or lack thereof) in the scene. Use larger ISO (800, 1600) for darker settings -- but know that you might see additional noise/grain.
Amazon sells tons of portable tripods that you can leave in your backpack. I have one from REI that I purchased years ago for $10. Be sure you also have an iPhone adapter for your tripod. I've had the best luck with this one: Mobile Phone Tripod Adapter